1. Field of the Invention
This invention relates generally to wireless communication systems and more particularly to supporting multiple wireless communication protocols within a wireless local area network using a long training sequence.
2. Description of Related Art
Wireless and wire lined communications are supported by current and legacy devices within existing networks and systems. Communication systems may include from national or international cellular telephone systems to, the Internet, and point-to-point in-home wireless networks. A communication system is constructed, and may operate, in accordance with one or more communication standards or protocols. Wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and the like.
The IEEE 802.11 specification has evolved from IEEE 802.11 to IEEE 802.11b to IEEE 802.11a and to IEEE 802.11g. Wireless communication devices that are compliant with IEEE 802.11b (standard 11b) may exist in the same wireless local area network (WLAN) as IEEE 802.11g (standard 11g) compliant wireless communication devices. Further, IEEE 802.11a (standard 11a) compliant wireless communication devices may reside in the same WLAN as standard 11g compliant wireless communication devices.
The different standards may operate within different frequency ranges, such as 5 to 6 gigahertz (GHz) or 2.4 GHz. For example, standard 11a may operate within the higher frequency range. One aspect of standard 11a is that portions of the spectrum between 5 to 6 GHz are allocated to a channel. The channel may be 20 megahertz (MHz) wide within the frequency band. Standard 11a also may use orthogonal frequency division multiplexing (OFDM). OFDM may be implemented over sub-carriers that represent lines, or values, within the frequency domain of the 20 MHz channels. The signal may be transmitted over many different sub-carriers within the channel. The sub-carriers are orthogonal to each other so that information may be extracted off each sub-carrier about the signal.
A communication system also may include legacy wireless devices. Legacy devices are those devices compliant with an earlier version of the standard, but reside in the same WLAN as devices compliant with a later version of the standard. A mechanism may be employed to ensure that legacy devices know when the newer version devices are utilized in the wireless channel to avoid a collision.
Thus, newer devices or components using current standards should have backward compatibility with already installed equipment within a network. These devices and components should be adaptable to legacy standards and current standards when transmitting information within the network. Legacy devices or components may be kept off the air or out of the network so as not to interfere or collide with information that they are not familiar with. For example, if the legacy device receives a signal or information supported by a newer standard, then the device should forward the information or signal to the appropriate destination without modifying or terminating the signal or information. Further, the received signal information may not react to the legacy device as if the legacy device is a device compatible with the newer standard.
For example, backward compatibility with legacy devices may be enabled exclusively at either the physical (PHY) layer or the Media-Specific Access Control (MAC) layer. At the PHY layer, backward compatibility is achieved by re-using the PHY preamble from a previous standard. Legacy devices may decode the preamble portion of all signals, which provides sufficient information for determining that the wireless channel is in use for a specific period of time, and avoid interference even though the legacy devices cannot fully demodulate or decode the transmitted frame(s).
At the MAC layer, backward compatibility with legacy devices may be enabled by forcing devices that are compliant with a newer version of the standard to transmit special frames using modes or data rates that are employed by legacy devices. These special frames contain information that sets the network allocation vector (NAV) of legacy devices such that these devices know when the wireless channel is in use by newer stations.
Mechanisms for backward compatibility may suffer from a performance loss relative to that which can be achieved without backward compatibility and are used independently of each other. Further, in standard 11a and 11g transmitters, only 52 subcarriers (−26 . . . −1 and +1 . . . +26) are filled with non-zero values even though an IFFT (inverse fast Fourier transform) of length 64 may be used. As such, sharp frequency-domain transitions may occur between zero subcarriers and non-zero subcarriers, which results in a time-domain ringing. This adversely affects a receiver's ability to detect a valid preamble transmission and requires the receiver to perform a channel estimate using the full fast Fourier transform (FFT) size.